U.S. Pat. No. 3,595,786 discloses an apparatus for treating fluids whereby a tabular treating agent is stacked in tubes suspended in a container through which the fluid is passed. The container comprises a manually adjustable weir which provides the desired height of the liquid level in the part of the container comprising the tubes with the treating agent. These tubes comprise at their lower ends axial slots through which the liquid may contact the treating agent, whereby an amount of the latter corresponding to the height of the liquid level in the container is contacted by the liquid.
This apparatus is designed for adjusting the liquid level in the container according to the flow rate of the liquid under more or less pressureless conditions such as used for sewage treatment. Variations of the pressure would add a further parameter influencing the liquid level in the container which can not automatically be compensated by a once adjusted weir. Thus the concentration of the treating agent in the liquid would depend on the pressure of the liquid. Further the simple shape of the weir does not guarantee a proportional relationship between the flow rate and the liquid level within the dissolution chamber over a wide range.
U.S. Pat. No. 4,293,425 discloses a device for chlorinating water of a swimming pool. A pump feeds water from the pool to a filter and back into the pool and the chlorinating device is arranged such that water coming from a point in the filter-pump circuit having elevated pressure enters a chamber in which it contacts and gradually dissolves pieces of chlorinating material and leaves the chamber to be fed back to the circuit at a point having a lower pressure. The chlorinating chamber is gas-tight, such that incoming water compresses the air entrapped in the chamber and the water is forced out from this chamber by the re-expanding air when operation of the pump is interrupted.
The amount of chlorinating material which is contacted by the water, pressed into the chlorinating chamber against the resistance of the air entrapped therein and thus the amount of dissolved material is dependent on the pressure of the incoming water since the height of the liquid level in the chamber depends thereon. Such devices may therefore not be used for liquid circuits which operate under variable pressure.
Other systems are known such as hydraulic ejectors which use the principle of a spray jet producing a pressure drop within a chamber which it traverses, thereby sucking in a fluid provided from a second supply conduit. Such ejectors produce a significant pressure drop in the main flow and additionally, the amount of agent sucked in from the second supply conduit depends on the pressure in the main flow which is not necessarily proportional to the flow itself. Moreover, this kind of system may only be used when the agent to be added to the main flow is in liquid condition.
Further, systems are known which use pumps for the side stream passing the dissolution chamber, whereby the operation of the pump may be controlled to provide constant concentration of the dissolvable agent in the main flow. Such systems, however, are expensive, consume energy and comprise a variety of moving parts which need regular maintenance.